1. Field of the Invention
The present invention relates to an apparatus that uses light with a wavelength of 300 nm or less as a light source. Such an apparatus includes an image-focusing optical system that can be installed in various apparatases, such as dn ArF excimer laser lithographic apparatus, ArF excimer laser CVD apparatus, and ArF excimer laser machining apparatus, etc. The present invention also relates to light-transmitting optical members, such as lenses, prisms and windows, etc., which con be incorporated into these optical systems. The present invention further relates to a projection exposure apparatus that uses light with a wavelength of 300 nm or less as a light source, and more particularly, to a projection exposure apparatus that uses an ArF excimer laser as a light source.
2. Discussion of the Related Art
Conventionally, reduction projection type exposure apparatuses known as "steppers" have been used in photolithographic techniques in which fine patterns of integrated circuits are exposed and transferred onto the surfaces of silicon wafers or the like. The optical systems of such steppers are constructed of an illumination optical system for uniformly illuminating the surface of a reticle (on which an integrated circuit pattern is drawn) with light from a light source, and a projection optical system for projecting and transferring the integrated circuit pattern of the reticle onto the surface of the wafer with a reduction ratio of one-fifth of the reticle pattern size, for example. Such apparatuses that use light to transfer integrated circuit patterns onto the surfaces of wafers are collectively referred to as "photolithographic apparatuses". With the increased integration of the LSI in recent years, a increase in the resolution of the transferred patterns on the wafer has become necessary. The resolution of the transfer pattern is proportional to the inverse of the wavelength of the light from the light source and to the numerical aperture of the projection optical lens system. Accordingly, a higher resolution can be obtained by increasing the numerical aperture and/or shortening the wavelength of the light source.
However, due to various limitations imposed on the lens manufacture, it is difficult to increase the numerical aperture of lenses. Accordingly, the wavelength of the light source has been reduced to increase the resolution. For example, in the light sources of steppens, the wavelength has been shortened to the g-line (436 nm), i-line (365 nm), and even further to KrF (249 nm) and ArF (193 nm) excimer laser beams. Especially, in order to manufacture the VLSIs, such as DRAMs with the memory capacities of 64 MB, 256 MB, 1 GB, 4 GB or greater, it is necessary to reduce "the line-and-space value" (which is an indicator of the resolution of steppers) to 0.3 .mu.m or less. In such cases, it is necessary to use ultraviolet or vacuum ultraviolet light with a wavelength of 250 nm or less, such as excimer laser beams, as a light source.
In general, optical glass materials that have been used in the lens members of the illumination optical system and the projection optical system of a stepper with a light source having a wavelength longer than that of the blind exhibits an abrupt drop in light transmissivity in the wavelength shorter than the i-line. In particular, almost all such optical glass materials cease to transmit light in the wavelength shorter than 250 nm. For this reason, silica glass, which has a high transmissivity at that short wavelength region and is suitable for obtaining optical members having an large diameter and high homogeneity in the refractive index, is effective as an optical glass material for use in the optical systems of steppers using excimer lasers as light sources. Furthermore, among other crystal materials, the uses of lithium fluoride crystals, magnesium fluoride crystals, and calcium fluoride crystals are conceivable. The calcium fluoride crystals are especially suitable, because the calcium fluoride crystals have a high transmissivity in the light region extending from ultraviolet to vacuum ultraviolet light and are also optically isotropic. Furthermore, the calcium fluoride crystals can be grown to a large crystal suitable for the manufacture of bigger lenses.
Conventional calcium fluoride crystals exhibit a high durability in light transmissivity with respect to a KrF excimer laser (a favorable characteristic for use with a KrF excimer laser) and are therefore usable for optical systems having a high power KrF excimer laser as a light source. The details of this property can be found in APPLIED OPTICS/Vol. 32, No. 29/6062-6075/(1992).
These two optical materials, silica glass and calcium fluoride, are indispensable for correcting chromatic aberrations and monochromatic aberrations in image-focusing optical systems using light with a wavelength of 300 nm or less as a light source, such as ArF excimer laser light
When silica glass and calcium fluoride crystals are used in the image-focusing optical system (such as the illumination optical systems or projection optical systems) of an ArF excimer laser lithographic apparatus (wavelength of 193 nm), the optical members need to have a high homogeneity in refractive index with little distortion and a high transmissivity in order to expose the integrated circuit pattern with a high resolution over a large area. For example, in projection optical systems, many lenses with different curvatures are necessary for correcting various aberrations. Therefore, the total optical path in the projection optical system may reach 1000 mm or greater in some cases. In illumination optical systems, although the total optical path is shorter than that for projection optical systems, the energy density of the illuminating ArF excimer laser light is high. Thus, a higher transmissivity is required in the lens members. To achieve a higher total transmissivity, each of the optical members need to have a sufficiently high bulk transmissivity.
The methods used to manufacture high-quality silica glass and calcium fluoride crystals used in the above-mentioned image-focusing optical systems are described below.
For silica glass, a vapor phase synthesizing method known as "direct method" is used. Silica glass manufactured using this method is particularly known as "synthetic silica glass". In this direct method, a silicon compound with a high purity, such as silicon tetrachloride, is used as a raw material. This raw material is hydrolyzed in oxygen-hydrogen flames, thus forming fine silica glass particles (soot). This soot is accumulated, melted, and is made transparent in a single, almost simultaneous process on the surface of a target that is rotated oscillated, and is pulled downward, thereby producing a silica glass ingot.
There have also been attempts to obtain silica glass with greater homogeneity by subjecting silica glass optical members produced by this synthesis method to a subsequent heat treatment at about 2000.degree. C. In this case, since desired physical properties are obtained by performing a separate heat treatment, such a process is referred to as "secondary" in contrast to the primal process by which the silica glass is synthesized. This method has an advantage in that the synthesized glass has fewer metal impurities and therefore a higher purity than molten quart glass obtained by the electrical melting or flame melting of powdered natural quartz. Accordingly, the resultant glass has a high transmissivity in the ultraolet region at wavelengths of 250 nm or less, enabling the manufacture of silica glass optical members with a larger diameter and greater homogeneity.
On the other hand, high-quality calcium fluoride crystals can be obtained by means of the crucible lowering method (known as "Bridgeman method" or "Stockberger method"). in this method, large-diameter, high-quality single crystals can be grown by utilizing the temperature distribution inside a furnace and by pulling the crucible downward.
It has been assumed that even for ultraviolet laser image-focusing optical systems, such as the illumination optical systems and projection optical systems, of ArF excimer laser lithographic apparatuses, it should be possible to use synthetic silica glass or calcium fluoride crystals obtained by the conventional manufacturing methods above in order to obtain sufficiently high transmissivity.
In calcium fluoride crystals, the absorption end at the short wavelength side is 124 nm, so that calcium fluoride crystals should, in principle, be usable for the light range extending from ultraviolet to vacuum ultraviolet. In other words, calcium fluoride crystals for use in an optical system for a 248 nm KrF excimer laser could also be used as is for an optical system using a 193 nm ArF excimer laser as a light source. In actuality, however, calcium fluoride crystal irradiated with a high energy light beam from an ArF excimer laser exhibit drastic deterioration of the transmissivity.
The maximum energy density of the laser beam illuminating the optical members used in the image-focusing optical system of a reduction type projection exposure apply is approximately 100 mJ.multidot.cm.sup.-2 /pulse. At such a high energy density, conventional calcium fluoride crystals show a transmissivity drop of approximately 8% or large per unit centimeter. This means a considerable amount of the radiation energy is absorbed in the crystal. Furthermore, heat is generated in the lens resulting from the conversion of light energy into heat energy by such energy absorption. This in turn results in variations in the refractive index and microscopic deformations of the shape of the optical member, making the optical member no longer usable.
Similar problems occur for optical members made of silica glass; e.g., damage to the optical member becomes a serious problem when the energy density of the light illuminating the optical member is high.